Dynamic Responses Analysis of a Building Structure Subjected to Ground Shock from a Tunnel Explosion

Dynamic responses of a multi-storey building without or with a sliding base-isolation device for ground shock induced by an in-tunnel explosion are numerically analyzed. The effect of an adjacent tunnel in between the building and the explosion tunnel, which affects ground shock propagation, is considered in the analysis. Different modeling methods, such as the eight-node equal-parametric finite element and mass-lumped system, are used to establish the coupling model consisting of the two adjacent tunnels, the surrounding soil medium with the Lysmer viscous boundary condition, and the multi-storey building with or without the sliding base-isolation device. In numerical calculations, a continuous friction model, which is different from the traditional Coulomb friction model, is adopted to improve the computational efficiency and reduce the accumulated errors. Some example analyses are subsequently performed to study the response characteristics of the building and the sliding base-isolation device to ground shock. The effect of the adjacent tunnel in between the building and the explosion tunnel on the ground shock wave propagation is also investigated. T.he final conclusions based on the numerical results will provide some guidance in engineering practice.     

Analysis of blast wave propagation inside tunnel

The explosion inside tunnel would generate blast wave which transmits through the longitudinal tunnel. Because of the close-in effects of the tunnel and the reflection by the confining tunnel structure, blast wave propagation inside tunnel is distinguished from that in air. When the explosion happens inside tunnel, the overpressure peak is higher than that of explosion happening in air. The continuance time of the blast wave also becomes longer. With the help of the numerical simulation finite element software LS-DYNA, a three-dimensional nonlinear dynamic simulation analysis for an explosion experiment inside tunnel was carried out. LS-DYNA is a fully integrated analysis program specifically designed for nonlinear dynamics and large strain problems. Compared with the experimental results, the simulation results have made the material parameters of numerical simulation model available. By using the model and the same material parameters, many results were adopted by calculating the model under different TNT explosion dynamites. Then the method of dimensional analysis was used for the simulation results. As overpressures of the explosion blast wave are the governing factor in the tunnel responses, a formula for the explosion blast wave over-pressure at a certain distance from the detonation center point inside the tunnel was derived by using the dimensional analysis theory. By comparing the results computed by the formula with experimental results which were obtained before, the formula was proved to be very applicable at some instance. The research may be helpful to estimate rapidly the effect of internal explosion of tunnel on the structure.     

Effect of Shock Wave on Fabricated Anti-Blast Wall and Distribution Law Around the Wall Under Near Surface Explosion

The loads of shock wave effect on fabricated anti-blast wall and distribution law around the wall were investigated by using near surface explosion test method and FEM.The pressure-time histories and variety law on the foreside and backside of the anti-blast wall were adopted in the tests of variety of different explosion distances and dynamites,as well as in the comparison between the test and numerical calculation.The test results show that the loads of shock wave effect on the anti-blast wall were essen-tially consistent with calculation results using criterion under surface explosion when explosion distances exceed 2 m,the distribution of overpressure behind wall was gained according to variety law based on small-large-small.It is also demonstrated that the peak overpressure behind wall had commonly appeared in wall height by 1.5--2.5 multiples,and the peak overpressures of protective building behind wall could be reduced effectively by using the fabricated anti-blast wall.     

Response of a transmission tower-line system at a canyon site to spatially varying ground motions

Collapses of transmission towers were often observed in previous large earthquakes such as the Chi-Chi earthquake in Taiwan and Wenchuan earthquake in Sichuan, China. These collapses were partially caused by the pulling forces from the transmission lines generated from out-of-phase responses of the adjacent towers owing to spatially varying earthquake ground motions. In this paper, a 3D finite element model of the transmission tower-line system is established considering the geometric nonlinearity of transmission lines. The nonlinear responses of the structural system at a canyon site are analyzed subjected to spatially varying ground motions. The spatial variations of ground motion associated with the wave passage, coherency loss, and local site effects are given. The spatially varying ground motions are simulated stochastically based on an empirical coherency loss function and a filtered Tajimi-Kanai power spectral density function. The site effect is considered by a transfer function derived from 1D wave propagation theory. Compared with structural responses calculated using the uniform ground motion and delayed excitations, numerical results indicate that seismic responses of transmission towers and power lines are amplified when considering spatially varying ground motions including site effects. Each factor of ground motion spatial variations has a significant effect on the seismic response of the structure, especially for the local site effect. Therefore, neglecting the earthquake ground motion spatial variations may lead to a substantial underestimation of the response of transmission tower-line system during strong earthquakes. Each effect of ground motion spatial variations should be incorporated in seismic analysis of the structural system.     

Numerical Simulation of Dynamic Response and Collapse for Steel Frame Structures Subjected to Blast Load

The progressive collapse of steel frame structures under the blast load was investigated using LS-DYNA. The multi-material Eulerian and Lagrangian coupling algorithm was adopted. A flu-id-structure coupling finite element model was established which consists of Lagrange element for simulating steel frame structures and concrete ground, multiple ALE element for simulating air and TNT explosive material. Numerical simulations of the blast pressure wave propagation, struc-tural dynamic responses and deformation, and progressive collapse of a five-story steel frame structure in the event of an explosion near above ground were performed. The numerical analysis showed that the Lagrangian and Eulerian coupling algorithm gave good simulations of the shock wave propagation in the mediums and blast load effects on the structure. The columns subjected to blast load may collapse by shear yielding rather than by flexural deformation. The columns and joints of steel beam to column in the front steel frame structure generated enormous plastic defor-mation subjected to intensive blast waves, and columns lost carrying capacity, subsequently lead-ing to the collapse of the whole structure. The approach coupling influence between struc-tural deformation and fluid load well simulated the progressive collapse process of structures, and provided an effective tool for analyzing the collapse mechanism of the steel frame structure under blast load.     

Numerical Simulation of Dynamic Response and Collapse for Steel Frame Structures Subjected to Blast Load

The progressive collapse of steel frame structures under the blast load was investigated using LS-DYNA.The multi-material Eulerian and Lagrangian coupling algorithm was adopted.A fluid-structure coupling finite element model was established which consists of Lagrange element for simulating steel frame structures and concrete ground,multiple ALE element for simulating air and TNT explosive material.Numerical simulations of the blast pressure wave propagation,structural dynamic responses and deformation,and progressive collapse of a five-story steel frame structure in the event of an explosion near above ground were performed.The numerical analysis showed that the Lagrangian and Eulerian coupling algorithm gave good simulations of the shock wave propagation in the mediums and blast load effects on the structure.The columns subjected to blast load may collapse by shear yielding rather than by flexural deformation.The columns and joints of steel beam to column in the front steel frame structure generated enormous plastic deformation subjected to intensive blast waves,and columns lost carrying capacity,subsequently leading to the collapse of the whole structure.The approach coupling influence between structural deformation and fluid load well simulated the progressive collapse process of structures,and provided an effective tool for analyzing the collapse mechanism of the steel frame structure under blast load.     

深埋圆形富水隧道应力与位移的弹塑性解

研究目的：推导获得深埋圆形富水隧道应力场与位移场分布的弹塑性解析解。创新要点：皋于弹性力学厚壁圆筒受均布压力的拉姆解答和Mohr—Coulomb屈服条件,推导了考虑渗流作用和应力释放时含衬砌深埋隧道的弹塑性解,并采用FLAC3D有限差分程序验证了其正确性。研究方法：通过理论分析建立考虑渗流作用和应力释放的含衬砌深埋圆形隧道的解析模型（图1）,然后推导了基于Mohr-Coulomb屈服准则的隧道同岩与衬砌渗流场、位移场和应力场计算的弹塑性解析公式,并通过数值模拟程序（图2）验证了推导公式的正确性。重要结论：基于前人研究皋础,推导获得了深埋富水隧道应力与位移场分布的理论解析解,并通过数值模拟方法验证了其正确性。该解析解一方面可以用于深埋高水头隧道的预设计,如预测隧道周围的水压力、应力场和位移场分布规律,另一方面可用于校核大型复杂数值模型的正确性。     

悬停时考虑地面效应的直升机的配平

在单旋翼涵道尾桨直升机的非线性全量飞行动力学数值模型的基础上,提出了求解配平问题的一种混合算法,研究了地面效应对配平的影响。以某直升机为算例,在无地面效应和考虑不同悬停高度时,分别进行了初始参数进行了配平,配平计算结果与地面效应的理论相符合。     

Ground-borne vibrations due to dynamic loadings from moving trains in subway tunnels

In this study, ground vibrations due to dynamic loadings from trains moving in subway tunnels were investigated using a 2.5D finite element model of an underground tunnel and surrounding soil interactions. In our model, wave propagation in the infinitely extended ground is dealt with using a simple, yet efficient gradually damped artificial boundary. Based on the assumption of invariant geometry and material distribution in the tunnel??s direction, the Fourier transform of the spatial dimension in this direction is applied to represent the waves in terms of the wave-number. Finite element discretization is employed in the cross-section perpendicular to the tunnel direction and the governing equations are solved for every discrete wave-number. The 3D ground responses are calculated from the wave-number expansion by employing the inverse Fourier transform. The accuracy of the proposed analysis method is verified by a semi-analytical solution of a rectangular load moving inside a soil stratum. A case study of subway train induced ground vibration is presented and the dependency of wave attenuation at the ground surface on the vibration frequency of the moving load is discussed.     

Splitting PMMA with Mini Cutting Explosives

In order to improve the present aviation ejection escape system, the application of explosion cutting technique to aviation escape system is proposed to weaken the strength of canopy before ejecting it. A series of mini cutting explosives are designed to investigate the process of splitting PMMA plate. The phenomenon of spallation in PMMA is observed. The effects of different parameters of mini cutting explosives on the cutting depth are obtained. Consequently the appropriate material of half-circular metal covers, explosive types and the ranges of charge quantities are determined. On the other hand, the cutting process of aviation PMMA plate by mini cutting explosives is simulated by means of nonlinear dynamic analysis code LS-DYNA. In finite element analysis,Arbitrary Lagrangian Euler (ALE) algorithm is used to depict the fluid property of high energy explosives. Continuous damage material model is used to simulate the complicate dynamic damage behavior of PMMA due to explosion shock waves. Only sliding contact option is defined to fulfill the fluid-structure interaction between explosives and PMMA plate by distributed parameter methods. Phenomenon of spallation observed in the experiment is presented in the simulation. The relationship between the penetration depth of PMMA plate and charge linear density obtained by numerical simulation agrees well with experimental result.     

Low frequency vibration tests on a floating slab track in an underground laboratory

Low frequency vibrations induced by underground railways have attracted increasing attention in recent years. To obtain the characteristics of low frequency vibrations and the low frequency performance of a floating slab track (FST), low frequency vibration tests on an FST in an underground laboratory at Beijing Jiaotong University were carried out. The FST and an unbalanced shaker SBZ30 for dynamic simulation were designed for use in low frequency vibration experiments. Vibration measurements were performed on the bogie of the unbalanced shaker, the rail, the slab, the tunnel invert, the tunnel wall, the tunnel apex, and on the ground surface at distances varying from 0 to 80 m from the track. Measurements were also made on several floors of an adjacent building. Detailed results of low frequency vibration tests were reported. The attenuation of low frequency vibrations with the distance from the track was presented, as well as the responses of different floors of the building. The experimental results could be regarded as a reference for developing methods to control low frequency vibrations and for adopting countermeasures.     

水洞试验中阻塞比对阻力测量影响的数值模拟

为了使表面形貌减阻效果的水洞试验结果可以模拟和预测自由边界的结果、使水洞试验中验证过的有效减阻措施可以成功运用到无限场的情况，该文采用当今最流行的数值模拟方法，基于FLUENT软件对有水洞壁的模型试验和自由边界的无限场模型进行模拟。结果表明，水洞试验的阻塞比不得超过25％，否则在水洞试验中验证的有效减阻措施将无法应用到自由边界的无限场。在水洞试验阻塞比的有效范围内，随着阻塞比的增大或来流速度的增大，表面摩擦系数随之增大。     

Reliability based multiobjective optimization for design of structures subject to random vibrations

Based on a multiobjective approach whose objective function （OF） vector collects stochastic reliability performance and structural cost indices, a structural optimization criterion for mechanical systems subject to random vibrations is presented for supporting engineer＇s design. This criterion differs from the most commonly used conventional optimum design criterion for random vibrating structure, which is based on minimizing displacement or acceleration variance of main structure responses, without considering explicitly required performances against failure. The proposed criterion can properly take into account the design-reliability required performances, and it becomes a more efficient support for structural engineering decision making. The multiobjective optimum （MOO） design of a tuned mass damper （TMD） has been developed in a typical seismic design problem, to control structural vibration induced on a multi-storey building structure excited by nonstationary base acceleration random process A numerical example for a three-storey building is developed and a sensitivity analysis is carried out. The results are shown in a useful manner for TMD design decision support.     

从做功看两类摩擦力的性质和做功特点

由于静摩擦力和滑动摩擦力在概念、存在条件和作用效果等方面都很相似,所以。通常人们都忽略了它们之间的差别,认为静摩擦力和滑动摩擦力是同一类力,把它们都归属为摩擦力,而实际上静摩擦力和滑动摩擦力是性质完全不同的两类力。本文从力学的基本原理出发,通过对静摩擦力和滑动摩擦力做功过程的理论分析和计算,给出静摩擦力和滑动摩擦力做功特点及做功过程能量转换差异,从而得出结论：静摩擦力和滑动摩擦力是两类性质不同的力,即静摩擦力是一种非耗散力而滑动摩擦力是一种耗散力。     

Surface settlements induced by tunneling in permeable strata: a case history of Shenzhen Metro

A case study of a significant surface settlement induced by tunneling in permeable strata with the shallow tunneling method is presented in this paper. The measurements of surface settlements along the excavation direction were first analyzed to highlight the impacts of groundwater seepage. Due to the groundwater inflow inside a double-arched tunnel, the surface settlement developed to a high level far beyond the measured crown settlement. The settlement-affected zone extended to 4 times the tunnel height ahead of the forefront heading and 1.5 times the tunnel height behind the hindmost heading. Consolidation resulting from high pore pressure change was considered to be the main mechanism for the large surface settlements. In addition, a 3D fluid-mechanical coupled numerical analysis was carried out to confirm the relationship between the significant surface settlement and pore pressure variation. This analysis reveals that lowering the permeability of the small pipe grouting zone, especially of the primary lining, could lessen the drop in pore pressure in the overlying strata, further reducing the total surface settlement. The numerical results also suggest that the transverse range of vertical displacement could be quite wide, and the settlement developed integrally from the tunnel crown towards the ground surface due to groundwater seepage. Moreover, the effect of advance drainage on surface settlement was investigated based on the same numerical model. Drainage with horizontal boreholes could considerably increase the safety of tunnel heading but had limited impact on surface settlement. Finally, the applications of pre-grouting and advance drainage measures were discussed for tunneling cases in permeable strata.     

Numerical simulation of water mitigation effects on shock wave with SPH method

The water mitigation effect on the propagation of shock wave was investigated numerically. The traditional smoothed particle hydrodynamics (SPH) method was modified based on Riemann solution. The comparison of numerical results with the analytical solution indicated that the modified SPH method has more advantages than the traditional SPH method. Using the modified SPH algorithm, a series of one-dimensional planar wave propagation problems were investigated, focusing on the influence of the air-gap between the high-pressure air and water and the thickness of water. The numerical results showed that water mitigation effect is significant. Up to 60% shock wave pressure reduction could be achieved with the existence of water, and the shape of shock wave was also changed greatly. It is seemly that the small air-gap between the high-pressure air and water has more influence on water mitigation effect.     

Analysis of tensioned membrane structures considering cable sliding

INTRODUCTIONTensionedmembranestructuresaremostsuitableforuseasroofstructureforavarietyofbuildingtypes,astheyprovidealight,elegant,andefficientstructurespanningoveralargeclearspace (Otto,1 973 ) .Examplesincludegymnasi ums,exhibitioncenters ,concertpavilio…     

Seismic response analysis of damper-connected adjacent structures with stochastic parameters

Dynamic response analysis of damper connected adjacent multi-story structures with uncertain parameters is carried out. A formula of the multi degree of freedom （MDOF） for the structure-damper system with stochastic parameters is derived. The uncertainties of mass and stiffness are taken into consideration firstly. The ground acceleration is represented by Kanai-Tajimi filtered non-stationary process. The mean square random responses of structural displacement and story drift are chosen as the optimization objective. The variations of mean square responses of top floor displacements and bottom story drifts in neighboring structures with the damper stiffness and damping coefficient are analyzed in detail. Through the parametric study, the acquiring optimum parameters of damper are regarded as numerical results. Then, a reducing order model of the MDOF system for adjacent structures with mean parameters is presented. The explicit expressions for determining optimal parameters of Kelvin model-defined damper which is used to connect adjacent single degree of freedom （SDOF） structures subjected to a white-noise excitation are employed to achieve the appropriate damper parameters, which are called theory results. Through a comparative study, it can be found that the theory values of damper parameters are consistent with the results based on extensive parametric studies. The analytical results can be obtained by using the first natural frequencies and the total mass of the adjacent deterministic structures with mean parameters. The analytical formulas can be used to find appropriate parameters of damper between adjacent structures for engineering applications. The performance of damper is investigated on the basis of mitigations of mean square random responses of inter-story drifts, displacements and accelerations in adjacent structures. The numerical results demonstrate the robustness of coupled building control strategies.     

Rocking of a rigid block freestanding on a flat pedestal

The seismic protection of objects contained within museums is a topic of great interest, especially with reference to how they are displayed or stored. This problem is the same as that of a large class of non-structural components, such as mechanical and electrical hospital and laboratory equipment that could lose their functionality because of earthquakes. Statues and ceramics simply supported on the floor represent a significant set of case. In some cases, like the Bronzes of Riace, isolation systems have been developed. However, in general museum exhibits are not equipped with devices capable of mitigating the oscillations induced by possible earthquakes. The case study of a marble statue placed on a freestanding squat rigid pedestal is examined. The system of algebraic differential equations governing the problem has been derived and included in an ad-hoc numerical procedure. It is shown that the insertion of a squat rigid body with low frictional resistance at the lower interface with the floor, and high frictional resistance at the upper interface with the artifact significantly reduces the amplitude of the rocking response. As a result the artifact rocks without sliding on the rigid base that slides without rocking with respect to the floor. The numerical analysis performed can be a tool to help in the choice of the optimal friction values in the surfaces of the flat block, designed as a simple isolation system.     

Analysis of tensioned membrane structures considering cable sliding

In routine design of tensioned membrane st ructures, the membrane is gen erally modeled using space membrane elements and the cables by space cable eleme nts, with no sliding allowed between the membrane and the cables. On the other h and, large deflections are expected and sliding between the membrane and the cab les is inevitable. In the present paper, the general finite element code ABAQUS was employed to investigate the influence of cable sliding on membrane surface o n the structural behavior. Three analysis models were devised to fulfill this pu rpose: (1) The membrane element shares nodes with the cable element; (2) The cab le can slide on the membrane surface freely (without friction) and (3) The cable can slide on the membrane surface, but with friction between the cable and the membrane. The sliding problem is modeled using a surface-based contact algorithm . The results from three analysis models are compared, showing that cable slidin g has only little influence on the structure shape and on the stress distributio ns in the membrane. The main influence of cable sliding may be its effect on the dynamic behavior of tensioned membrane structures.